linux-kernel - Re: [PATCH v3 0/6] Add support for privileged mappings

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Message-ID: <20160725095012.GC15864@arm.com>
Date:	Mon, 25 Jul 2016 10:50:13 +0100
From:	Will Deacon <will.deacon@....com>
To:	Mitchel Humpherys <mitchelh@...eaurora.org>
Cc:	iommu@...ts.linux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jordan Crouse <jcrouse@...eaurora.org>,
	Jeremy Gebben <jgebben@...eaurora.org>,
	Patrick Daly <pdaly@...eaurora.org>,
	Pratik Patel <pratikp@...eaurora.org>,
	Thomas Zeng <tzeng@...eaurora.org>
Subject: Re: [PATCH v3 0/6] Add support for privileged mappings

On Fri, Jul 22, 2016 at 01:39:45PM -0700, Mitchel Humpherys wrote:
> On Fri, Jul 22 2016 at 05:51:07 PM, Will Deacon <will.deacon@....com> wrote:
> > On Tue, Jul 19, 2016 at 01:36:49PM -0700, Mitchel Humpherys wrote:
> >> The following patch to the ARM SMMU driver:
> >> 
> >>     commit d346180e70b91b3d5a1ae7e5603e65593d4622bc
> >>     Author: Robin Murphy <robin.murphy@....com>
> >>     Date:   Tue Jan 26 18:06:34 2016 +0000
> >>     
> >>         iommu/arm-smmu: Treat all device transactions as unprivileged
> >> 
> >> started forcing all SMMU transactions to come through as "unprivileged".
> >> The rationale given was that:
> >> 
> >>   (1) There is no way in the IOMMU API to even request privileged mappings.
> >> 
> >>   (2) It's difficult to implement a DMA mapper that correctly models the
> >>       ARM VMSAv8 behavior of unprivileged-writeable =>
> >>       privileged-execute-never.
> >> 
> >> This series rectifies (1) by introducing an IOMMU API for privileged
> >> mappings and implements it in io-pgtable-arm.
> >> 
> >> This series rectifies (2) by introducing a new dma attribute
> >> (DMA_ATTR_PRIVILEGED) for users of the DMA API that need privileged
> >> mappings which are inaccessible to lesser-privileged execution levels, and
> >> implements it in the arm64 IOMMU DMA mapper.  The one known user (pl330.c)
> >> is converted over to the new attribute.
> >> 
> >> Jordan and Jeremy can provide more info on the use case if needed, but the
> >> high level is that it's a security feature to prevent attacks such as [1].
> >
> > This all looks good to me:
> >
> > Acked-by: Will Deacon <will.deacon@....com>
> >
> > It looks pretty fiddly to merge, however. How are you planning to get
> > this upstream?
> 
> Fiddly in what way?  Do you mean in relation to "dma-mapping: Use
> unsigned long for dma_attrs" [1]?  I admit I wasn't aware of that
> activity until Robin mentioned it.  It looks like it's merged on
> next/master, shall I rebase/rework on that and resend?

Fiddly in that it touches multiple subsystems. I guess routing it via
the iommu tree (Joerg) might be the best bet.

Will